1. Field of the Invention
The present invention relates to a Nb--Ti-base alloy superconducting magnetic shield. More particularly, the present invention relates to a seamless magnetic shield in a hollow body form comprising a multilayer composite laminate composed of an Nb--Ti-base alloy layer and a metal layer having a high conductivity.
2. Description of the Related Art
In theory, a superconductor in the form of a hollow vessel having a bottom, or a hollow body not having a bottom, can provide a good shielding of the inside thereof from a magnetic filed until the strength of an external magnetic field reaches a certain value. This is because a superconducting shielding current flows in the superconductor in a close loop and generates a magnetic flux which cancels the total amount of flux close loof coused by an external field, and thus a magnetic field in the opposite direction is formed and the inside of the vessel becomes a low filed region. Accordingly, various magnetic shields having the above-described shape have been proposed, and some are in practical use. The prior art will now be described.
(1) An Nb block is cut to form a cylinder 1 not having a bottom, as shown in FIG. 2(a), or a tetragonal hollow body 1a as shown in FIG. 2(b), and these bodies are used for a magnetic shielding of a sensor section of a AQUID magnete meter used for the measurement of a small magnetic field. For example, in this case, a superconducting shielding current 2 flows in a closed loop in a direction perpendicular to the direction of an external magnetic field 3, to thereby form a magnetic field, and thus reduce the external magnetic field.
(2) As shown in FIG. 3, a superconductor in a sheet or foil form is formed into a doughnut-like disk 4 having a hole in the center thereof, and disks having the same shape are laminated one on top of the other to form a cylinder 5. Alternatively, the lamination is conducted while sandwiching a normal conductor 6 having the same shape as the disk, to form a cylinder 7. In this case, since a superconducting shielding current 2 flows only through superconducting disks, the cylinder 7 can shield only external fields parallel to axis.
(3) As shown in FIG. 4, a superconductor 8 in a square plate or foil form is cylindrically bent, and the meeting edges are connected to each other. In this case, a superconducting shielding current 2 forms a closed loop through a seam 9. This connection is usually conducted by welding, soldering, and contact bonding.
The method shown in FIG. 2, wherein a block material of a Nb or Nb--Ti is cut into a hollow body, has a poor yield, and further, to improve the magnetic shielding property, pinning centers which prevent a movement of quantized fluxes, i.e., maintain a zero resistivity, for example, a fine precipitate of a normal conductor, or a dislocation network must be distributed in the material. Especially for Nb--Ti, this requires that a cold working be conducted with a high percentage working before and after a heat treatment, under suitable conditions, but it is difficult to subject a block material to cold working with a high percentage working, and the cold working with a high percentage becomes more difficult after the block material is cut into a hollow body. A high percentage working, makes it difficult to prepare a hollow body capable of shielding a large space, from a block material.
To stabilize the superconducting property, it is necessary to dispose a metal having a high conductivity, such as Cu or Al, in a metallically bonded state around the superconducting material, such as Nb or Nb--Ti. Further, preferably the thickness of the superconducting layer is greatly reduced to about 100 .mu.m or less, to stabilize the superconducting property. In the block material, however, a sufficient percentage working cannot be attained because of its large size, and thus a sufficient metallic bond cannot be obtained between the metal having a high conductivity and the superconducting material. Therefore, it is obvious that a formation of a multilayer with a superconducting layer having a thickness of about 100 .mu.m or less is almost impossible.
The technique shown in FIG. 3 is effective when the size of the disk is relatively small. Although the technique has no shielding effect against transverse-fields, an excellent effect can be attained against longitudinal fields, since there is no seam to cause a lowering of the property in a loop through which a superconducting shielding current flows. In this case, it is relatively easy to form a multilayer with a metal having a high conductivity, and further, the thickness of the superconducting layer can be easily reduced to about 100 .mu.m or less by deposition or sputtering. Nevertheless, since the material in the hole provided in the central portion is removed, the yield is poor when the proportion of the hole is high. Further, the size of the disk cannot be increased to more than the size of the plate used as the material. Moreover, a limitation exists in that it is relatively difficult to increase the size of the cylinder in the axial direction.
The technique shown in FIG. 4 provides an excellent yield of the material when preparing a cylinder, and further, facilitates an increase in the size of the cylinder in the axial direction. In this technique, however, a seam must appear, and this causes the superconducting property in the seam to become much lower than in the seamless portion. Bolting is unsatisfactory in the formation of a metallic bond, and welding causes a loss of pinning centers, such as precipitates and dislocations formed in the seamless portion; these bring a remarkable lowering of the property. With respect to soldering, there should exist a contact resistance which decreases the shielding effect.
Further, in the case of a multilayer sheet prepared by alternately laminating a metal having a high conductivity and a superconducting metal, it is almost impossible to conduct a satisfactory seaming.